This invention relates to improvements in investment casting and more particularly to improvements in an investment casting procedure where the heat utilised to melt the wax-type pattern and to sinter the ceramic mould is provided by microwave energy.
In investment casting first a model of the article to be moulded, usually from molten metal, is wax injected into a reverse engineered mould, or fabricated from a wax type pattern material. The pattern material may be natural or synthetic wax, polystyrene, or blends of various waxes, thermoplastic materials usually, but not exclusively, including fillers such as adipic acid and plasticizers. As used herein and in the appended claims the expression “wax type pattern material” is intended to include all such heat fusible pattern materials suitable for use in a “lost wax” moulding procedure. Typically a number of similar models are attached to a “sprue” to form a “tree” of the pattern material and the whole is coated several times typically, but not exclusively, with ceramic slurry and sand type material. The ceramic coating is then dried to provide a hard mould around the “wax type pattern material”. The pattern material is melted out and the ceramic “shell” is sintered and molten metal is then poured into the “shell” void. When the metal has hardened the ceramic shell can be removed.
Investment casting using conventional sources of heat is a very lengthy and expensive procedure. It has been proposed e.g. in British Patent No. 1 457 046 to use microwave energy, thereby shortening the procedure and making it more economical. However the principal problems encountered in investment casting arise from differential expansion and contraction of the different materials involved when being heated up and cooled down. A particular danger is that if the pattern material cannot escape fast enough from the ceramic shell when being melted it may crack the shell due to its expansion. British Patent No. 1 457 046 offers as a solution to this problem the inclusion in the ceramic slurry of a so called “lossy material” which will induce a rapid melting of the pattern material adjacent to the shell. The solution however is imperfect especially when moulding articles of such a shape that the pattern material can only escape from the ceramic shell through a restricted bottle-neck, sprue or pour cup. If the material of the sprue is not melted first, or is imperfectly melted, the escape path for the rapidly expanding material within the shell is blocked with the result that the shell may be cracked.
It has been proposed in Japanese patent publication JP56117857 to use a resin type mould that can be melted out of the shell without deformation or cracking. This solution however is imperfect as it relies on placing the resin mould into a container of water allowing the water to penetrate through the honeycomb sections of the mould by capillary action. By this technique the volume of water will be generally constant throughout the mould where exposed above the water surface, i.e. there will be no gradient of susceptor content throughout different areas of the mould. Moreover this type of resin moulding cannot be used on high specification finishes of the cast components (such as aero engine blades) without a further polishing process, due to the manufacturing type of process of resin moulds, which do not produce a smooth finish to the casting.
A principle object of the present invention is to resolve these problems by providing a differential melting characteristic for wax pattern material in different parts of the mould, such that material in a sprue or other restricted opening will melt before material in other areas of the mould upstream of the opening. Thus when the latter material in turn becomes molten its escape route is not blocked and it can exit the mould while expanding without endangering the mould shell. The current virgin wax patterns, which must be used in the production of engine blades, can be used in accordance with this invention.